1. Field of the Invention
The present invention relates in general to the prevention of a no-load operation of a microwave oven, and more particularly to a method of preventing the no-load operation of the microwave oven in which the presence of a load or food in a heating chamber is discriminated according to a resistance variation of gas and temperature sensors which sense a variation in a gas amount and a temperature variation in the heating chamber, respectively, and, if the presence of no load in the heating chamber is discriminated, a heating operation is automatically stopped to prevent a damage due to heating under a no-load condition and an unnecessary power consumption.
2. Description of the Prior Art
Referring to FIG. 1, there is shown a schematic circuit diagram illustrating a construction of a conventional microwave oven. As shown in this drawing, the conventional microwave oven comprises a load driving circuit 2 connected in parallel between a primary coil of a high-voltage transformer (HVT) 1 and a power source to perform a cooking operation, a heater 3 for applying radiant heat to a heating chamber (not shown) in which a load or food is placed, a microwave generator 4 connected to a secondary coil of the high-voltage transformer 1 to generate a microwave, and a plurality of relays RY1-RY3 for turning on/off the high-voltage transformer 1, the heater 3 and the load driving circuit 2, respectively. The relay RY1 is a main relay for controlling the output of the microwave generator 4, the relay RY2 is a grille relay for controlling a temperature in the heating chamber and the relay RY3 is an auxiliary relay for controlling an oven lamp OL, a turntable motor TM and a fan motor FM in the load driving circuit 2.
Referring to FIG. 2, there is shown a schematic circuit diagram illustrating a circuit for controlling the operation of the conventional microwave oven. As shown in this drawing, the control circuit includes a microcomputer 6 for controlling the entire system operation, a gas sensor 7 for sensing a gas amount generated in the cooking operation, a temperature sensor 8 for sensing a temperature in the heating chamber, and a key input unit 5 for selecting a desired function according to a user's selection. The microcomputer 6 controls the relays RY1-RY3 in response to output signals from the gas and temperature sensors 7 and 8 to control the high-voltage transformer 1, the heater 3 and the load driving circuit 2.
The gas sensor 7 has one side for inputting a supply voltage Vcc and the other side connected to one side of a fixed resistor R, the other side of which is connected to a ground terminal. Also, the gas sensor 7 has a resistance varied with a variation in the gas amount. The supply voltage Vcc is divided by the varied resistance of the gas sensor 7 and a resistance of the fixed resistor R and then applied to the microcomputer 6. Similarly, the temperature sensor 8 has one side for inputting the supply voltage Vcc and the other side connected to one side of a fixed resistor R', the other side of which is connected to the ground terminal. Also, the temperature sensor 8 has a resistance varied with a variation in the temperature. The supply voltage Vcc is divided by the varied resistance of the temperature sensor 8 and a resistance of the fixed resistor R' and then applied to the microcomputer 6.
The operation of the conventional microwave oven with the above-mentioned construction will hereinafter be described with reference to FIGS. 3 to 5.
FIG. 3 is a flowchart illustrating a manual cooking operation of the conventional microwave oven. First, when a manual cooking function is selected by the key input unit 5 according to the user's selection, the microcomputer 6 turns on the main relay RY1 and the auxiliary relay RY3 according to the selected manual cooking function. As being turned on, the auxiliary relay RY3 drives the oven lamp OL, the turntable motor TM and the fan motor FM in the load driving circuit 2. As the main relay RY1 is turned on, the microwave generator 4 is driven by the high-voltage transformer 1, so as to apply the microwave to the heating chamber. As a result, the manual cooking operation of the microwave oven is advanced. Then, when a predetermined time period TA has elapsed, the manual cooking operation of the microwave oven is stopped regardless of the operation of the gas sensor 7.
FIG. 4 is a flowchart illustrating a grille automatic/manual cooking operation of the conventional microwave oven. First, in the case where a grille automatic/manual cooking function is selected by the key input unit 5 according to the user's selection, the microcomputer 6 turns on the auxiliary relay RY3 according to the selected grille automatic/manual cooking function. As being turned on, the auxiliary relay RY3 drives the oven lamp OL, the turntable motor TM and the fan motor FM in the load driving circuit 2. Then, the microcomputer 6 compares the present temperature C. sensed by the temperature sensor 8 with a predetermined value CA. If the present temperature C. sensed by the temperature sensor 8 is greater than the predetermined value CA as a result of the comparison, the microcomputer 6 determines that the temperature in the heating chamber is sufficiently high. As a result, the microcomputer 6 maintains the auxiliary relay RY3 at its ON state for a predetermined time period TB under the condition that it does not turn on the grille relay RY2. On the contrary, if the present temperature C sensed by the temperature sensor 8 is smaller than the predetermined value CA as a result of the comparison, the microcomputer 6 determines that the temperature in the heating chamber is not sufficiently high. As a result, the microcomputer 6 further turns on the grille relay RY2 to allow the heater 3 to apply the radiant heat to the heating chamber. Then, when the predetermined time period TA has elapsed, the grille automatic/manual cooking operation of the microwave oven is stopped.
FIG. 5 is a flowchart illustrating an automatic cooking operation of the conventional microwave oven. First, in the case where an automatic cooking function is selected by the key input unit 5 according to the user's selection, the microcomputer 6 turns on the auxiliary relay RY3 according to the selected automatic cooking function to drive the oven lamp OL, the turntable motor TM and the fan motor FM in the load driving circuit 2. Then, the microcomputer 6 maintains the auxiliary relay RY3 at its ON state for a predetermined time period (about 18 seconds) to discharge the remaining smell in the heating chamber of the microwave oven. When the predetermined time period has elapsed, the microcomputer 6 turns on the main relay RY1.
As the main relay RY1 is turned on, the microwave generator 4 is driven by the high-voltage transformer 1, so as to apply the microwave to the heating chamber. As a result, the automatic cooking operation of the microwave oven is advanced. The gas begins to be generated in the heating chamber as the automatic cooking operation of the microwave oven is advanced. The resistance Rs of the gas sensor 7 is reduced as the generated gas is increased in amount. At this time, a combined resistance .DELTA.G (Ro/Rs) is increased, where Ro is the resistance of the fixed resistor R. Then, the microcomputer 6 is operated in such a manner that the microwave from the microwave generator 4 can be supplied to the heating chamber until the combined resistance .DELTA.G reaches a predetermined value K. When the combined resistance .DELTA.G reaches the predetermined value K, the heating operation is further performed for an additional heating time period TC and, then, the automatic cooking operation of the microwave oven is stopped.
However, the above-mentioned conventional microwave oven has a disadvantage in that the heating operation is continuously performed even upon the presence of no load or food in the heating chamber, resulting in a damage of the heating chamber and the associated components. Also, the continuous heating operation results in an unnecessary power consumption.